The present invention relates to electrical components, and is to be applied in the structural design of a gas-filled surge arrester that is provided with an external short-circuit device.
Short-circuit device provided with external short-circuit devices are common both in two-electrode and in three-electrode surge arresters. They function to protect the surge arrester in long-term load situations. Generally, such a short-circuit device contains a structural element that can melt at higher temperatures, with the aid of which the two electrodes, or the center electrode and one or both end electrodes, are short-circuited.
In a conventional two-electrode arrester of this type, a spring contact and an arrester contact are arranged electrically parallel to the two electrodes, the spring contact being held at a distance from the arrester contact by a spacer that is adjacent to the insulator of the arrester and that softens at increased temperatures. This spacer is made of a glass-fiber-reinforced plastic that is dimensionally stable at normal operating temperature and that softens at increased temperatures, e.g. polycarbonate. Such a two-electrode arrester is described in European Patent 0 548 587.
In addition, a conventional external short-circuit device for two- and three-electrode surge arresters is described in WO 87/06399. This short-circuit device is composed of a two-layer clamp made of a thermostatic bimetal, the two layers of the clamp passing, over at their free ends, into a monostable snap element. In the event of an overload, one part of the snap element functions to bridge the two external electrodes of the arrester. In two-electrode arresters, the bimetal clamp sits on the insulator of the arrester.
In U.S. Pat. No. 4,984,125, a conventional three-electrode arrester, an external short-circuit device is described that is composed of a bracket, made of an open ring, that is placed on the center electrode, the one end of the ring forming, together with arms shaped at the sides, a short-circuiting link. The free ends of the two arms are held at a radial distance from the end electrodes of the arrester by means of a spacer that is made of a meltable insulating material and that is arranged between the center electrode and the short-circuiting link. The free ends of the two arms can thereby be provided with special contacts. Instead of using a spacer between the center electrode and the short-circuiting link, the mentioned contacts can also be coated with an insulating coating. French Patent No. 2 670 624 describes a similarly configured external short-circuit device that can be used both for three-electrode and for two-electrode arresters. In this device, the free ends of the two arms of the short-circuiting link are bent in such a way that they abut the circumferential edge of the respective electrode, the free ends having in the area of the point of contact an insulating coating that can melt when heat is applied.
The object of the present invention is to configure the short-circuit device in such a way that it extends radially past the surge arrester as little as possible, ensures a safe contacting, and is easy to install.
In order to achieve this object, it is provided according to the present invention that the spacer is arranged between the center part of the short-circuiting link and the insulator.
Such a configuration of the short-circuit device makes use of the conventional design of a short-circuit device such as that described in U.S. Pat. No. 4,984,125, but the bracket, in this context, is placed not on an electrode, but ratherxe2x80x94in a manner such as described in French Patent No. 2 670 624)xe2x80x94on the insulator located between two electrodes. In this way, the short-circuiting link is arranged without potential. Given cramped installation conditions (magazine or cartridge), this can prove to be advantageous insofar as no disturbing short circuit can occur in case of contact of one side of the short-circuiting link with a contact terminal that conducts potential. Since the spacer is arranged between the short-circuiting link and the insulator, the spacer can be of relatively thin construction; in addition, a relatively inexpensive thermoplastic plastic material can be used for the spacer, since the insulator heats up with a delay relative to the electrodes that are to be short-circuited.
A surge arrester having short-circuit device configured according to the present invention is thus distinguished by a design that is very compact and that saves a great deal of space with regard to the insertion of a larger number of such arresters into cartridges, the short-circuit device being able to be made of one or two parts and to be easily adapted to various arrester geometries due to its simple configuration. The short-circuit device can also be retrofitted; it can also be exchanged after being blown, as long as the actual arrester is still intact.
The short-circuit device configured according to the present invention can be used both for two-electrode and for three-electrode arresters. In three-electrode arresters, each of the two insulators present in such arresters is to be equipped with a short-circuit device of the present configuration.
The open snap-on ring of the new short-circuit device can be made of a heat-resistant plastic or of a resilient (spring-like) metal that conducts less well electrically (e.g. spring steel or a copper alloy), a metallic short-circuiting link that conducts very well electrically (e.g. made of copper) being placed onto the one end of the open ring or being fastened there in some other way. Preferably, however, the open ring is made of the same material as the actual short-circuiting link, and is configured in one piece with the latter, the open ring being provided at the one end with two side bars, shaped at the sides, for the formation of the short-circuiting link. In order to improve the contacting, these side bars can be bent slightly at their free ends.
A plastic part that can be melted may be used as a spacer for the short-circuit device. In the same way, a spacer made of a soldering material can also be used. These spacers can, for example, have the form of a rivet fastened to the center part of the link. Alternatively, a plastic clip placed on the center part of the link can also be used.